1. Field of the Invention
The present invention relates to a particle beam irradiation apparatus and a particle beam therapy system for performing treatment of a cancer or the like by use of a particle beam.
2. Description of the Related Art
A particle beam therapy system is a system for performing treatment of a cancer, a tumor, and the like, by making good use of the feature, of a particle beam, that “it selectively demonstrates an effect at the deeply inner part of a body”; the contents of the technology thereof can be seen from various literatures (e.g., International Publication No. WO2006-082651/pamphlet).
The fact that “a particle beam selectively demonstrates an effect at the deeply inner part of a body” is based on the nature, of a particle beam, that it has a Bragg peak. As represented in FIG. 1 of International Publication No. WO2006-082651/pamphlet, in the case of a small-mass radiation beam such as an X-ray or a gamma ray, among various kinds of radiations, the relative dose thereof becomes maximum at a body portion near to the surface of a body and decreases as the depth from the body surface increases. In contrast, in the case of a large-mass particle beam such as a proton beam or a carbon beam, the relative dose thereof has its peak value at a position that is deep in a body and at which the beam stops, i.e., immediately before the range of the particle beam ends. The peak value is called a Bragg peak BP.
Briefly speaking, the Bragg peak suggests that “the intracorporeal place where a particle beam selectively demonstrates an effect is as narrow as a point”; however, in order to irradiate a particle beam onto an irradiation target in such a way that the dose distribution is uniform on the overall irradiation target, there is performed “enlargement of the irradiation field” of a particle beam.
The irradiation-field enlargement includes enlargement in the traveling direction (Z direction) of a particle beam and enlargement in a direction (XY-plane direction) perpendicular to the z direction. In this specification, according to International Publication No. WO2006-082651/pamphlet, the enlargement in the Z direction is referred to as “depth-direction irradiation-field enlargement”, and the enlargement in the XY-plane direction is referred to as “transverse-direction irradiation-field enlargement”.
Transverse-direction irradiation-field enlargement utilizing a typical passive-method is exemplified by the scatterer method. In the scatterer method, by irradiating a particle beam onto a scatterer in the particle beam irradiation unit of a particle beam irradiation apparatus, the particle beam is expanded in the transverse direction, and the middle portion, of the particle beam, in which the distribution of the dose is uniform is cut off and irradiated onto a target site. In the case where a single scatterer cannot sufficiently enlarge the uniform-dose portion, two scatterers may be utilized so that the uniform-dose portion is enlarged; this is referred to as the double scatterer method.
Transverse-direction irradiation-field enlargement utilizing a typical active-method is exemplified by the pencil beam scanning method. In the pencil beam scanning method, a particle beam is scanned in the XY plane by use of a deflection electromagnet provided at the upstream side of the particle beam irradiation unit of a particle beam irradiation apparatus and the irradiation position of the particle beam is moved as the time elapses, so that a wide irradiation field is obtained. In this method, a uniform dose distribution can be obtained by making neighboring irradiation spots of small-diameter pencil beams appropriately overlap one another. The pencil beam scanning method includes the raster method in which a particle beam is scanned in a continuous manner with respect to the time and the spot method in which a particle beam is scanned in a step manner with respect to the time. In this method, a particle beam, which is referred to as a pencil beam and, in general, has a small diameter, is directly irradiated onto a target site; however, the diameter of the pencil beam may slightly be enlarged by use of a thin scatterer.
There has been considered a method which is an intermediate between the passive method and the active method. Transverse-direction irradiation-field enlargement utilizing a typical intermediate method is exemplified by the Wobbler method. In the Wobbler method, a particle beam is scanned in the shape of a donut by use of two deflection electromagnets provided at the upstream side of the particle beam irradiation unit of a particle beam irradiation apparatus and the particle beam, which is scanned in the shape of a donut, is irradiated onto a scatterer, so that the transverse irradiation field is enlarged.
Next, irradiation-field enlargement in the depth direction will be described. As described above, the width of a Bragg peak BP in the irradiation direction of a particle beam is narrow; the irradiation-field enlargement in the depth direction denotes enlargement of the irradiation-direction width of a Bragg peak BP. The Bragg peak BP whose irradiation-direction width has been enlarged is referred to as a Spread-Out Bragg Peak SOBP.
Depth-direction irradiation-field enlargement utilizing a typical passive method is exemplified by a method utilizing ridge filter or a range modulator. In each of a ridge filter and a range modulator, the thickness of the material of an energy modulator is modulated in the irradiation direction of a particle beam. In each of a ridge filter and a range modulator, the energy of a particle beam is reduced in accordance with the modulated thickness so that the energy is changed in accordance with the modulated thickness; as a result, a particle beam, in which many kinds of intensity-changing energies are mixed, is irradiated onto an irradiation target. Because the range of a particle beam changes in accordance with the intensity of the energy, particle beams having different ranges can be irradiated onto an irradiation target. Such a passive depth-direction irradiation-field enlargement method makes it possible to obtain a Spread-Out Bragg Peak SOBP whose width is enlarged in the irradiation direction; however, the width of the Spread-Out Bragg Peak SOBP is constant and cannot be changed in the transverse directions, i.e., in the X-axis direction and the Y-axis direction that are perpendicular to the irradiation direction of a particle beam.
Accordingly, in the case where a ridge filter or a range modulator is utilized, a device named “bolus” is also utilized. As illustrated in FIG. 2 of International Publication No. WO2006-082651/pamphlet, a bolus is an energy modulator obtained by performing machining for each patient in accordance with the distal form (changing form, in the depth direction, of a site to be treated); a bolus is made of polyethylene or wax. The use of a bolus makes it possible to make the Bragg peak BP coincide with the distal form while irradiating a uniform irradiation dose over the XY plane.
In general, a ridge filter has a shape obtained by combining approximately triangular prisms, as illustrated in FIG. 2 of Japanese Patent Application Laid-Open No. 2007-75245; a ridge filter has a cross-sectional shape as illustrated in FIG. 3 of Japanese Patent Application Laid-Open No. 2007-75245; a ridge filter is integrated in an irradiation system, as illustrated in FIG. 1 of Japanese Patent Application Laid-Open No. 2007-75245.
As disclosed in Japanese Patent Application Laid-Open No. 2007-75245, there has been reported a problem that, in a particle beam therapy system utilizing a ridge filter, scattering becomes insufficient. In the case where the particle beam is a proton beam, because being relatively light, the particle beam is sufficiently scattered by air and an irradiation subject, whereby, spatially, the particle beams can sufficiently be mixed with one another in the irradiation field. However, in the case where the particle beam is a particle beam of relatively heavy particle, because scattering is not likely to occur, no uniform irradiation-dose distribution is obtained at the end of the range, and there is produced a dose valley at a position that corresponds to the ridge of a ridge filter. In other words, there has been a problem that, the dose distribution in the vicinity of the range-end position becomes a striped periodical distribution, as illustrated in FIG. 4(2) of Japanese Patent Application Laid-Open No. 2007-75245.
In order to form a uniform dose distribution at the range end of an irradiation field, Japanese Patent Application Laid-Open No. 2007-75245 discloses an apparatus in which, during irradiation of a particle beam, a ridge filter is driven in the directions that are perpendicular to the advancing direction of the particle beam so that there is solved the problem, posed when the ridge filter is utilized, that scattering becomes insufficient.
It is true that, as a prior art, there has been proposed, as disclosed in Japanese Patent Application Laid-Open No. 2007-75245, that a ridge filter is mechanically wobbled through translation or rotation so that a uniform dose distribution is effectively achieved.
However, for example, as illustrated in FIG. 1 of Japanese Patent Application Laid-Open No. 2007-75245, a ridge filter is installed in the vicinity of a patient; thus, there has been a problem that driving the ridge filter causes noise and hence the patient is made to sense discomfort or anxiety.